1. Technical Field
The technology described herein is generally related to the field of integrated circuits and, more particularly, to driver circuits for light emitting diode (“LED”) chains.
2. Description of Related Art
LEDs are known act as a source of emitted light for a wide variety of applications. LEDs are known to provide many advantages over incandescent and fluorescent illumination because of their long operating life, high efficiency, light weight, and low profile. LED light output is proportional to current therethrough. Problems can be particularly egregious for portable devices where battery output gradually decreases with time of use. Therefore, an LED driver circuit is needed that is relatively immune to small voltage fluctuations.
Moreover, white light LEDs are particularly convenient for applications such as backlighting liquid crystal display (“LCD”) screens. The LEDs are often configured as a set of serial connected LEDs, sometimes referred to in the art and hereinafter as “LED chains,” “LED sets,” “LED banks,” or the like. Lightweight, battery-powered devices, such as mobile computing and communications devices—e.g., personal digital assistant, cellular telephone, electronic book, and the like—may use LEDs as a back light or side light. It is important to provide LED driver circuitry in which a constant current is provided to each LED to provide adequate lighting and to minimize flickering on the screen.
Furthermore, it is known that white light LEDs have relatively high threshold voltages for turn on, sometimes higher than the battery nominal output voltage. Therefore, DC-to-DC power supply booster circuits may be employed. U.S. Pat. No. 6,628,252 (Hoshino et al.) shows a known manner LED DRIVE CIRCUIT. A booster circuit is provided for boosting battery output voltage, describing a means for generating a constant current to an LED. U.S. Pat. No. 6,586,890 (Min et al.) shows a known manner LED DRIVER CIRCUIT WITH PWM OUTPUT. U.S. Pat. No. 6,359,392 (He) describes a HIGH EFFICIENCY LED DRIVER. Basically, these LED driver circuits generate a LED drive potential by boosting a battery voltage.
FIG. 1 (PRIOR ART) is a schematic diagram illustrating a passive inductor, “LEXT,” passive discharge capacitor “CEXT,” type DC-DC boost converter 101—used in commercial products such as the Model 2287 integrated circuit manufactured by the assignee herein—for driving a chain 102 of white LEDs 103-1, 103-2, 103-N using a pulse width modulator (“PWM”) 108 technique, wherein current feedback substantially constantly adjusts the power to the LED chain (see also, e.g., Min et al., incorporated herein by reference). The voltage level at the output node 104 connected to the LED chain 102 is established by a reference voltage “VREF” applied to one terminal, node 107, of a comparator operational amplifier 105, also sometimes referred to in the art as the “error amplifier,” having an appropriately sized resistor “REXT” 106, with the LED chain 102 connected to the other terminal of the amplifier. The output voltage at node 104 adjusts until the loop through the 101 controls the current in the LED chain 102 such that the current there through is defined as,I=VREF/REXT,where VREF is a regulated voltage powered from VIN.
However, with such a scheme, in a device having a plurality of sets of LEDs, each chain would require a separate such booster circuit 101. Power efficiency—battery life—is reduced by each booster circuit employed. This also is cost inefficient.